Was it ever stated in Scripture before Paul that Christ's blood atoned for all the sins of the whole world?
No. According to the context, Scripture never plainly stated before Paul that Christ’s blood had atoned for all the sins of the whole world. Such truth may have existed in latent language, but it was not openly declared until Paul’s revelation.
Les teaches from Philippians 1 that the letter was written to a Gentile congregation, and he highlights a new body of truth never mooted before. He asks whether Christ’s earthly ministry spoke of a Gentile body of Christ, whether the Holy Spirit would indwell individual believers, or whether Scripture said Christ’s blood atoned for the whole world’s sins. His answer: not a word; it was latent, but not plainly stated until Paul.